U.S. Pat. No. 5,328,475 entitled "Simplified Safety Syringe with Retractable Self-biased Needle" issued to the same inventor of this application disclosed a safety syringe including: a hollow needle normally held in a front portion of a syringe, a plunger generally cylindrical shaped and slidably held in the syringe for injection use, a coupling member retained in the plunger and engageable with a biasing socket recessed in a rear needle portion of the hollow needle and with the biasing socket generally formed as a conical shape having a longitudinal conical axis inclined from a needle axis of the needle when normally fixed in the syringe with the needle axis aligned with a syringe axis longitudinally formed in a center line of the syringe, whereby upon retraction of the plunger and the needle coupled to the plunger, with the biasing socket of the needle forcibly coupled with the coupling member, into a bore portion in the syringe, the needle will be automatically inclined as the biasing socket of the needle is restored by the coupling member retained in the plunger to prevent an outward protruding of the retracted needle from the syringe for preventing its pricking to the others.
However, the plunger (31) secured on the coupling member (30) will require a big volume in order to efficiently surround the arrowhead portion (301) on the coupling member (30) and to engage the conical base portion (303) of the coupling member (30). Since the safety syringe is always made as disposable and will be disposed once being used for hygienic reason, such a big plunger, which is always made of resilient rubber materials, will waste money and will also increase a burden for waste disposal and treatment on a viewpoint of environmental protection.